Obtaining the best laser phaco fragmentation results involves striking a balance between two opposing goals. The first opposing goal is to cut as much of the volume of the crystalline lens into suitable pieces as possible, particularly the cataract-hardened parts of the lens, so that as little ultrasound energy and mechanical manipulation of the lens as possible is required to remove the lens. Of necessity, this means making incisions as close as possible to the lens capsule, and particularly the posterior capsule where, for some types of cataracts, a hard, difficult-to-remove “posterior plate” resides as mentioned in 1) Kamori, K and Mochiizuki, M, J Cataract Refract Surg. Vol. 36, No. 1, January, 2010, pp. 9-12 and 2) Buratto et al, “Phacoemulsification: Principles and Techniques,” published by SLACK Inc., Thorofare, N.J.). Cutting the posterior plate into pieces would provide a large benefit to the surgeon since it minimizes the risk of damaging the posterior capsule during the mechanical manipulation of the plate to break it into pieces or bring it into position for application of the ultrasonic phaco tip.
The second opposing goal is to have the procedure be as safe as possible by keeping a sufficient safety margin between the outermost extent of the pattern of laser incisions and the anterior and posterior capsule. The safety margin must account for all sources of error, including the most important sources of error, such as: 1) errors in measuring the positions of the anterior and posterior capsule and 2) errors in placement of laser shots by the beam guidance system.
For capsulotomies, a similar balance is required. The capsulotomy is cut as a series of stacked rings of individual laser pulses. The pulses within a ring and the z spacings between rings are spaced such that the photodisruption which occurs at each pulse has the composite effect of generating a smooth, continuous cylindrical cut. If the position of the anterior lens capsule at the desired position of the capsulotomy were known perfectly, a cylinder of very small edge height would be required; i.e., the edge height could be little more than the thickness of the lens capsule. Making the cylindrical cut with a small edge height is beneficial because it minimizes bubble formation under the lens capsule or in the anterior chamber of the eye. The formation of bubbles under the capsule can interfere with the laser cut and result in capsulotomies that are not completely cut and have residual tissue bridges between the capsulotomy “button” (circular region of anterior capsule which is removed) and the remaining lens capsule which must be manually torn. Anterior chamber bubbles can obstruct or interfere with the laser as the laser traverses the bubbles to cut parts of the cornea or lens in later parts of the laser procedure. Small edge heights for the cylindrical cut also reduce the surgery time. However, the requirement for small edge heights must be balanced against the risk that some or all of the incisions will miss the lens capsule and cut instead in the lens fibrous body or in the anterior chamber due to slight errors in the measurement of the lens capsule position or the positioning of the laser beam when making the incisions. Obviously, missing all or part of the lens capsule necessitates later manual tearing of some or all of the capsule—which negates some or all of the advantage of using the laser itself and which may also increase the risk of an anterior capsule tear when the surgeon tries to connect two parts of a laser cut with a manual tear to complete the capsulotomy.
Corneal incisions such as LRIs, as with the laser capsulotomy incisions, have stringent accuracy and precision requirements. While the arcuate incisions must cut deep into the eye (typically 90% of the 500-700 u.ro thickness of the cornea), it is undesirable to cut all the way through the cornea. In addition, the amount of astigmatism correction resulting from a cut is a function of the depth of the cut.
Another factor in obtaining good clinical outcomes in cataract surgery concerns the centration of the capsulotomy. In conventional cataract surgery, the capsulorhexis is centered on the patient's dilated pupil to the best of the surgeon's ability. Note that the pupil is used for centering since the pupil is the only readily available, visible landmark available to the surgeon. After the cataractous lens has been removed, the IOL is placed into the capsule so that the IOL's optic is centered as well as possible on the capsulorhexis as shown in FIG. 1. Centering the IOL's optic on the capsulorhexis helps to prevent the IOL from tilting, relative to the axis of the eye, as the edge of the anterior lens capsule adheres to the outer part of the optic and becomes fibrotic over time. With a poorly centered IOL, the capsular edge may be partially on and partially off the IOL's optic and the fibrosis will tend to pull one side of the IOL forward more than the other side, causing the lens to tilt with a resulting increase in aberrations in the images the IOL forms on the retina.
Although the foregoing IOL centration method is widely used, it has the disadvantage that it frequently results in a shift in the optic axis of the eye following cataract surgery. The shift is due to the fact that although the optical axis of the eye generally passes close to the center of the undilated pupil, the pupil generally dilates somewhat asymmetrically and the center of the dilated pupil is decentered with respect to the undilated pupil, and thus off the optic axis as described in Merchea, et al, Invest Ophthalmol Vis Sci 2005; 46: E-Abstract 4357. The IOL, therefore, is similarly off center relative to the original optic axis of the eye, and the misalignment between the optic axis of the cornea and the optic axis of the off center IOL causes increased aberrations in the images formed by the cornea and off center IOL, thus reducing the patient's visual acuity.
In order to place laser incisions within the cornea and lens, the positions and shapes of the anterior and posterior cornea and lens surfaces need to be determined. This is accomplished in conventional laser cataract surgery by use of an imaging or biometric device built into the laser system. Most often, the device is based on OCT (Optical Coherence Tomography) as described in Maxine Lipner, “What's ahead Femto technology changing the cataract landscape”, Eye World, Vol. 13, No. 33, Mar. 24, 2011, p. 19 or a Scheimpflug camera-based imaging system as disclosed in U.S. Patent Application Publication No. US 2010/0022995, the entire contents of which are incorporated herein by reference). Such systems make longitudinal sectional images of the eye; each image captures the position of the intersection of the cornea and lens with an illuminated vertical plane of light which generally passes through a diameter of the eye, i.e., includes the optic axis of the eye. The pupil position can be found from the same image, or with the assistance of a separate camera of known position and optical characteristics pointing directly into the eye. A single longitudinal sectional image may be used to allow a user of the laser to target the position of the capsulotomy and laser phaco fragmentation pattern as well as corneal incisions by using a computer mouse or other pointing device to manipulate software reticules superimposed on the longitudinal sectional image as shown in FIG. 2. This method forces the surgeon to interrupt the surgery to perform the time consuming task of manually targeting the various positions mentioned previously. The method also suffers from the lack of repeatability inherent in manual processing of images and manipulation of reticules to allow the surgeon to select the position and size of the laser phaco fragmentation pattern and capsulotomy. Since the method relies on the surgeon's skill in judging exactly where the anterior and posterior cornea and lens surfaces are within the slightly fuzzy images, it is not ideal for deciding on the optimal tradeoff between the need to cut fragmentation pattern very close to the capsule and still allowing an adequate safety margin. Furthermore, if the optic axis of the lens is tilted with respect to the axis of the laser, the tilt will be visible when a longitudinal sectional image passes through the plane defined by the axes of the laser and tilted crystalline lens, but in a longitudinal sectional image perpendicular to this plane, no tilt will be observable as will be explained in more detail below. Thus, if a laser phaco fragmentation pattern is placed on the basis of a single longitudinal sectional image, there is no way to ensure that tilt is properly accounted for. The presence of tilt may cause a fragmentation pattern which appears to “fit” into the crystalline lens in a particular longitudinal sectional image, when in fact the pattern, extends outside the boundaries of the lens, i.e., the pattern would cut through the lens capsule when the full three dimensional shape of the tilted lens is taken into account.
Alternatively, in conventional laser cataract surgery, the longitudinal sectional image or images may be processed automatically to find the position of edges representing the curved anterior and posterior surfaces of the cornea and lens in a laser-defined coordinate system. Examples of such automatic processing can be found in U.S. Provisional Patent Applications Nos. 61/228,506 and 61/455,178 and U.S. Patent Application Publications Nos. 2010/0004641 and 2010/0022995, the entire contents of each of which is incorporated herein by reference. If numerical curve fitting is applied to the edge points corresponding to a particular one of the eye's four optical surfaces, for example using a RANSAC curve fitting algorithm, (see, Peihua Li and Xianzhe Ma, “Robust Acoustic Source Localization with TDOA Based RANSAC Algorithm”, in Emerging Intelligent Computing Technology and Applications, edited by De-Shuang Fluang et al, Springer-Verlag, Berlin, 2009), a mathematical representation of the position of the surface within the longitudinal sectional images can be found. By repeating the process, a two dimensional mathematical representation, in the form of an arc of a circle, of each of the four surfaces can be obtained as shown in FIG. 3. (It should be noted that the term “arc” is meant to mean a curve with a more or less constant radius of curvature. However, the arc may refer to a section of an ellipse, a small section of a parabola, etc., within our usage.) Although not shown, the pupil position could also be derived automatically from the longitudinal sectional image or from an image from a camera pointing at the eye and the capsulotomy be centered by the laser system at the center of the pupil.
The computer image processing and curve fitting mentioned previously improves on the manual method in that the positions of the anterior and posterior cornea and lens surfaces can be found more accurately and repeatably and a necessary safety margin between the laser fragmentation pattern and lens capsule can be imposed automatically and precisely by the software. Despite the advantages mentioned above, the method still suffers from the problem of lens tilt. In addition, both the manual and automatic versions of this method are subject to large errors due to lens tilt as explained below.
In the methods mentioned above, the position of the laser capsulotomy is still, of necessity, at the center of the pupil, with the limitations of that type of positioning. All the foregoing methods, which involve a single planar sectional image of the eye, suffer from the possibility that the lens is tilted with respect to the axis of the laser optics. There are still alternate methods, such as disclosed in U.S. Provisional Patent Applications Nos. 61/228,506 and 61/455,178 and U.S. Patent Application Publications Nos. 2010/0004641 and 2010/0022995, that utilize two or more longitudinal or nearly longitudinal sectional images of the eye to reconstruct limited 3D models of the eye, which suffer from the same problem of not explicitly accounting for lens tilt. Such tilting may be the result from a slightly off-center docking of the laser system to the eye. (The laser system must be “docked” to the eye to hold the eye stationary and in a known position and orientation with respect to the coordinate system of the laser and the laser's built in eye measuring system.) This is conventionally accomplished by placing a specially designed circular suction ring on the eye at or near the limbus. The suction ring is docked or fastened to the laser in a rigid, defined manner to hold the eye at a fixed position relative to the laser. As shown in FIGS. 4A-B, if the suction ring is applied asymmetrically, i.e. centered with respect to the center of the limbus, the eye will be held by the docking device at an angle, tilted with respect to the laser axis.
As schematically shown in FIGS. 5A-C, a three-dimensional shot pattern has been placed within a three-dimensional representation of the lens, based on a single longitudinal sectional image of the eye as shown in FIG. 5A. Note that in FIGS. 5A-C, the lens capsule surrounding the lens is shown in gray and the shot pattern in pink. As shown in FIG. 5A, the shot pattern appears to fit within the lens capsule in this view. However when the lens and the embedded shot pattern are rotated by 45° (FIG. 5B) or 90° (FIG. 5C) around the Z axis, it is apparent that the shot pattern does not completely fit within the lens capsule. Thus, basing the placement of the laser phaco fragmentation pattern on a single longitudinal sectional image of the eye can lead to errors in such placement. Thus, to ensure that the shot pattern completely fits within the lens capsule, it is necessary to know the three-dimensional geometry of the lens, and in particular, the amount with which the lens is tilted with respect to the axis of the laser and biometric system. Although not shown, a similar problem exists with the placement of a capsulotomy or LRI on the basis of a single longitudinal section image of the eye.
FIGS. 5A-C illustrate the general problem that in order to place laser incisions in the cornea, or cut a laser capsulotomy of minimal edge height, or to cut a laser phaco fragmentation pattern within the crystalline lens, the three-dimensional shape and position of the target tissue must be known. In order to address this problem, an accurate determination of the three-dimensional shape and position of target tissue and the tilt of the lens relative to laser and biometric system coordinate system need to be known in order to properly center the capsulotomy, position the laser phaco fragmentation pattern appropriately within the lens and place corneal incisions correctly within the cornea, leaving appropriate safety margins with respect to preventing damage to nearby tissue.